In a mobile system of this kind, both voice and signalling data are transmitted in a digital manner by modulating a carrier frequency. Each carrier supports n users whose bit bursts are grouped to form a time division frame (TDMA). The system has a given number K of usable frequencies which are differently distributed in the different cells depending on their traffic, up to a maximum of m frequencies per one very high traffic density cell, for a total of m.times.n possible simultaneous speech (physical channels). Thanks to the cell subdivision, the same frequencies are assigned to more cells far enough among them to avoid interference, thus obtaining a high spectrum effectiveness. One of the features foreseen in digital mobile systems to mediate speech quality is that of frequency hopping.
According to this technique, during the speech, the time slot assigned to one user hops on a different frequency at each frame, while maintaining unchanged the frame structure. More specifically, the time slot of one user goes from a frequency (or carrier) to another one among those assigned to the base station (SRB) according to a predetermined law, but remaining in the same position inside the frame of n time slots.
In this way each speech can be time distributed on all the frequencies assigned to the SRB, and the result is that attenuation and distortion effects presented by one transmission frequency shall concern only a reduced number of frames of each user, and their negative effects shall be distributed among all users and not concentrated only on some of them.
It has been proposed to make the switch required by the frequency hopping directly by radio frequency or in base band.
In the first case, considering for instance the reception aspect, the base station foresees a plurality of receivers connected to a combining circuit receiving all frequencies assigned to the SRB from an antenna. Single receivers vary their frequency according to the frequency hopping algorithm and therefore each receiver always produces at the output the same n time slots assigned to the same n users which are routed in the known manner towards decoding units, and from there to the switched telephone network. The same takes place of course for transmission.
This solution is disadvantageous since it requires a broadband coupling among the antenna and each single receiver preventing a selective coupling. Furthermore, frequency synthesizers of the receivers has to be very quick, with switching time in the range of one hundred nanoseconds to perform the necessary frequency changes from a time slot and the subsequent one.
In case of baseband switching (interconnection), it is possible to use selection couplers (narrow band) among the combining circuit and each receiver/transmitters operating at fixed and constant frequency. The correct connection of each receiver/transmitter is carried out through a dynamic interconnection matrix which changes its configuration according to the frequency hopping algorithm to connect each receiver/transmitter with (time by time) correct process nodal point of the station controller.
A multiplexing matrix performing these interconnections constitutes however a centralized device involving a very high number of connections, and therefore problems exist with cost, reliability and difficulty of installation problems.
It is therefore a critical component which should be redundant (that is foreseen as double). In case of a maximum equipment of m frequencies, the matrix should have (m.times.n.times.2).sup.2 cross points. This dimension should increase if it is requested to exchange through the same way to control data among nodal points. A separate connection should be foreseen for controlling data among nodal points and the station controller should be necessarily centralized.
One of the aims of the present invention is to overcome the above mentioned troubles of the previous technique, and in particular to simplify and optimize the interconnection of a plurality of receiver/transmitters among them, or between the station controller and the different nodal points which must be interconnected in this base station, in a system performing in base band the switching required by the frequency hopping technique.
Another aim of the invention is to realize an interconnection system, which carries in a flexible manner the data from and to the station control center, among receiver/transmitters, process nodal points and also the possible supervision, maintenance and data collection systems. Furthermore the system according to the invention has the advantage of being easily redundant.